Combine many dependabot PRs into one

#4135 explored a scripted approach (running client side) of combining multiple PRs into one.

The motivation is that the sheer volume of dependabot PRs creates a weekly multi-hour merge chore, as well as obscuring real PRs by real people - both of which are terrible.

https://github.com/dependabot/dependabot-core/issues/1190 looks like it won't happen any time soon.

https://www.hrvey.com/blog/combine-dependabot-prs looks very promising and I think we should give it a try instead of my previous script attempt. I'd like to verify:

• that, as seen in #4135, we can have the release notes updates automatically. We'd need to:
  • enable merge commits in this repo, and make sure we use them when merging the combined PR
  • (I think) update our release-drafter workflow triggers, so that pull request events trigger release drafter. Even if not required, we should probably do that anyway.
• that merge conflicts are not too big a problem. We'll likely seem them occurring with changes in adjacent lines in build.gradle files, which may or may not be a nuisance.
  • if merge conflicts are a problem, I have written a dirt-simple git merge driver script which seems to work well enough for build.gradle files - perhaps we could explore using this.

Start by reviewing #4135 and the release-drafter workflow referenced in the issue. Test whether combining Dependabot PRs with merge commits updates release notes correctly, then evaluate merge conflicts in the build.gradle files; done means the workflow is reliable and conflicts are manageable.